The present invention relates to plant for drying any of a variety of materials including vegetable materials such as wood, coffee, cocoa, tobacco, fodder, etc. and animal materials such as leather, fish, cheese, etc.
More particularly, the invention concerns plant for drying materials, particularly timber, by air circulation, comprising a drying chamber having air inlet and air outlet ports for the circulation of air in the chamber, and a refrigerating unit for the dehumidification of the air, said refrigerating unit comprising a refrigerant fluid circuit including a compressor, means defining an air flow path through the refrigerating unit including a fan for forcing air through the said inlet port, a dehumidification evaporator located in the path of flow of air from the outlet port of the drying chamber, and a condenser located in the path of inlet air flow to the inlet port to restore to the inlet air flow at least part of the heat extracted from the outlet air by said dehumidification evaporator.
Plants of the type specified above have been available commercially for a number of years all over the world. Such plants are based on the principle that the drying air is circulated in a closed circuit which includes the drying chamber and the refrigerating unit, the air issuing from the drying chamber being dehumidified by refrigeration and the current of air thus dehumidified being reheated and reintroduced into the chamber. The drying process is carried out over several days, the relative humidity (R.H.) of the air in the drying chamber being lowered progressively, thereby stimulating the emergence of moisture from the interior to the surface of the material, where the moisture is removed by the circulating air.
Drying processes using such plants led to significant advantages over drying systems which were previously in use, as regards the speed and economy of the drying treatment, especially as applied to timber. In fact it has become possible to dry out timber in this way in a relatively short period of time, reducing its residual moisture content to 16-20%, depending on he species. Unfortunately, however, any further reduction of the residual moisture encounters many difficulties and requires long periods of time, and a residual moisture content of 6-8%, which many users of timber are now seeking, is almost impossible to achieve, especially for hardwoods such as oak, teak and afromosia, in thicknesses greater than about 60 mm. Indeed such drying would require, in the first place, such a long processing time as to make the system uneconomic, and in the second place the residual humidity of the circulating air in the chamber would have to be reduced to such low values that the wood would tend to become "cemented" (a term used to denote occlusion of the pores in the surface layers of the timber), with resultant cracks and distortion. Moreover, apart from this, prolonged periods of drying may easily lead to mould growth.
It is possible to augment the degree of drying of timber by means of high temperature steam treatment, but this involves considerable expenditure on plant and operating costs, so that such methods have hitherto been used only by large concerns. The conditions for this treatment are very critically dependent on the various types of wood. One of the attendant risks is that of discoluration, which it is absolutely essential to avoid for such varieties of wood as mahogany, walnut, rosewood, teak and mansonia, where colour is the most valuable attribute commercially. Another risk is that of collapse of the fibres, and flaws in the material caused by too sharp a rise in temperature. Furthermore, contact with high temperature steam can lead to shrinkage and a resulting loss of thickness.
An object of the present invention is to provide a process for drying material such as timber to an extremely low residual moisture content, even to within the range of 6-8%, and at high speeds, using plant of low cost which is inexpensive to operate.